Diversity and Lifestyle in the Rotifera

Broad questions regarding community assembly and lifestyle evolution remain unanswered. To answer these questions I used rotifers, common primary consumers present in inland waters. Rotiferan presence in nearly all freshwaters makes them an ideal study system to address questions of metacommunity ecology. Additionally, rotifers possess diverse lifestyles, including sessile, swimming, colonial and solitary forms, allowing them to be used as models of lifestyle evolution. In Chapter 1, I address metacommunity ecology by focusing on inland waters of the Chihuahuan Desert as a study system and investigated rotifer community assembly therein through two published works, Brown et al., 2020 and 2021. These studies used species present/absence data for rotifers from sites throughout the Chihuahuan Desert collected by the Walsh lab over a 20-year period. The first of these papers investigated the general patterns of rotiferan richness across this desert and looked at the influence of scale on them. Overall, I found hotspots of rotifer species richness that become less distinct at broader scales, where some hotspots may only appear at the regional level. I found the highest rotifer richness in springs (n=175) followed by lakes (n=112) and then rockpools (n=72). I found that sampling effort was linked to observed richness per site, and that distance was linked with beta dissimilarity at small spatial scales. Rotifer communities were found to be highly nested at these sites. To interpret broad patterns of rotifer diversity, richness across the Chihuahuan Desert was predicted using empirical Bayesian kriging, pooling sites at a variety of spatial scales. In Brown et al. (2021), I used subset of the dataset used in Brown et al. (2020) to investigate rotifer community assembly in temporary waters. I found that in these habitats rotifers are assembled into their communities stochastically. Additionally, I analyzed environmental parameters associated with the presence of particular rotifer species, finding several features such as hydroperiod and conductivity to be important, accounting for 12% of the total variation in community composition. Additionally, I found that richness was highest in the habitats with the greatest amount of aquatic vegetation. In Chapter 2, investigate the advantages of coloniality in rotifers. Many animals form colonies, although the reasons why they do this is unclear. For rotifers, one possible explanation is that colonies may provide the individual members of the colony with an energetic advantage. One way such an advantage may manifest is through lower respiration rates of individuals in colonies with more members, which would cause colony respiration rate to scale allometrically with colony size. Additionally, genome size may be directly tied to metabolism through the metabolic hypothesis of genome size and could also be used as a related character for further validation. To address these questions, rotifer respiration rates were measured using a Loligo microplate and microplate reader. I measured rates within the known range of rotiferan respiration rates reported in the literature. The findings on colony size allometry were mixed; Sinantherina socialis respiration scaled isometrically with colony size while Lacinularia flosculosa and Conochilus hippocrepis scaled allometrically. Additionally, I examined the traits that may be associated with allometric scaling of colony respiration across all colonial taxa with published respiration rates.To do this I utilized a hierarchical mixed regression model, and found that several features, including colony shape, presence of extrazooidal structures, and an unattached lifestyle influence respiration scaling. In Chapter 3, I explore coloniality by looking at how genome size relates to lifestyle in gnesiotrochan rotifers. Genome size is related to metabolism in some animals, such as in birds and mammals. I investigated whether genome size was associated with particular rotifer lifestyles such as coloniality and sessility. Genome sizes were measured by flow cytometry, using Propidium Iodide staining, and Drosophila melanogaster as a genome size standard. Genome sizes found for gnesiotrochan rotifers (0.05 – 0.16 pg) were similar to those of ploimid rotifer (0.06 – 0.46 pg). I found that genome sizes differed significantly depending on lifestyle, i.e., genome size was smaller in motile and solitary rotifers and larger in sessile and colonial rotifers. Overall, I found that rotifers assemble stochastically and with patterns that vary by scale for rotifer communities of the Chihuahuan Desert. Both rotifer genome sizes and allometric scaling factors suggest that coloniality in rotifers does seem to be related to metabolism, suggesting that an energetic advantage for coloniality may exist. Expanding the area investigated beyond the Chihuahuan Desert and incorporating other regions would help to determine whether the patterns I found can be applied to other biomes. Likewise, expanding the number of allometric scaling factors and genome sizes measured across colonial rotifers would enhance the reliability of these conclusions. These studies provide preliminary understandings of rotifer diversity and community assembly in one region, the Chihuahuan Desert, as well as supporting the energetic advantage hypothesis of coloniality